There is an exception. Those flying basic economy will be allowed to use online check-in only if they qualify for a full-sized, carry-on bag, or if customers indicate that they are checking a bag and pay the checked bag service charge, according to the United's website.

Already, basic economy prevents passengers from choosing, changing or upgrading seats. They cannot sit with their group or family, bring a full-sized carry-on bag on board, change flights or earn premier qualifying credits and lifetime miles.

Nonetheless, many customers are opting for basic economy to save $15 to $20 on their one-way ticket, multiple reports have said.